In Positron Emission Tomography (PET) an annihilation event is identified by time coincidence between the detection of two 511 keV, annihilation photons in two different detectors located around the patient. If two detectors A and B are hit coincidentally by full-energy photons, the detectors estimate a Line-of-Response (LOR) along which an annihilation has occurred. The position of the annihilation (and therefore the position of the radioactive source) along this line is not known, but existing technology reconstructs the image of the original radioactivity distribution from the large set of LORs identified by detector pairs in coincidence.
A large fraction of the 511 keV photons emitted during the decay and annihilation process undergo scatter before exiting the patient's body, mainly Compton scatter, during which they lose energy. 511 keV energy photons that reach the detectors are called “true” or “unscattered” photons. Photons with energy lower than 511 keV at the detectors are called “scattered” photons. A time coincidence of two detected photons is called a “coincidence event.” A true coincidence event is an event in which both detected photons have 511 keV energy. A scatter coincidence event is an event in which one or both detected photons are scattered photons.